Semiconductor components are used in a variety of portable electronic devices including laptop computers, cellular phones, personal digital assistants, video games, video cameras, etc. In these applications the devices may be operated without being connected to an external Alternating Current (AC) power source. Here a battery may be used to provide the power. However, operating the devices in this mode depletes charge from the battery such that they should be recharged. When recharging the batteries, it is desirable to quickly recharge them without an excessive power loss during the recharge operation so the devices can be used in a portable mode as quickly as possible.
FIG. 1 is a circuit schematic of a prior art converter circuit 10. What is shown in FIG. 1 is a resistor 12 having a terminal connected to a noninverting input terminal of an amplifier 14 and a terminal connected to the inverting input terminal of amplifier 14. The noninverting input terminal of amplifier 14 and the terminal of resistor 12 connected to the inverting input terminal of amplifier 14 are coupled for receiving an input signal at node or input terminal 16. Amplifier 14 has an output terminal 13 which provides a voltage signal VIR12 that is representative of a current IR12 flowing through resistor 12. The other terminal of resistor 12 is commonly connected to a system load 20 and to a drain terminal of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) 22. MOSFET 22 has a gate terminal coupled for receiving a Pulse Width Modulation (PWM) control signal from a PWM control circuit 24 and a source terminal commonly connected to the drain terminal of a MOSFET 26 and to a terminal of an inductor 28. The other terminal of inductor 28 is commonly connected to a non-inverting input terminal of an amplifier 30 and to a terminal of a resistor 32. The other terminal of resistor 32 is commonly connected to an inverting input terminal of amplifier 30 and to a capacitor 34 and a battery 36 to form an output terminal 38 at which an output voltage VOUT appears. Amplifier 30 has an output terminal 31.
In operation, current IR12 flows into node 18. A portion IL20 of current IR12 flows to system load 20 and a portion ID22 of current IR12 flows to the drain terminal of MOSFET 22 from which a portion IR32 flows through resistor 32. A voltage VIR12 appears at output terminal 13 and a voltage VIR32 appears at output terminal 31. Thus, converter circuit 10 includes two current loops where one of the current loops measures input current IR12 and the other measures, for example, a battery charging current IR32. Transmitting battery charging current IR32 through two sense resistors, i.e., resistors 12 and 32, increases the amount of power dissipated from converter circuit 10 making it inefficient.
Accordingly, it would be advantageous to have a method and circuit for determining a charging current that operates efficiently. It would be of further advantage for the method and circuit to be cost efficient to manufacture and operate and to be reliable.